I've got this shimano 600 EX rear derailleur that I'd love to use, but somehow the outer part of the cage got warped and the chain makes contact with it. It's aluminum so I can't really bend it back. I kinda doubt these kind of parts are available any more, especially when you can get a whole one for pretty cheap..

Yes, indivivual parts were offered but, as you say, finding sombody who has one may be difficult. Who says you can't straighten them? I do it fairly often, with varying degrees of success depending on the bend. They're a lot like out of true wheels, with a very high probablity of success no matter how large the bend, provided it's gradual. It's the small, sudden bends that are tricky.

Sorry to hear it broke. So, are we talking 600EX or New 600EX? Short, medium of long cage? If you don't know, just post the part number, which should be stamped on the back of the parellogram. The cage attaching to the pivot or the other side or both? The reason I'm asking is because many Shimano derailleurs used cages with identical geometry. Expanding the selection of donor derailleurs will increase your odds of finding a replacement.

It's an RD-6208... the "new" 600EX. looks like short cage to me, pulleys are ~47mm apart. I'd actually love to swap out the short cage for a long cage, but just finding a replacement would be pretty awesome

so i discovered, even though i broke the cage in half, i could still bolt it back on, and I guess i bent it enough so the chain doesn't drag any more. When the pulley gear bolts are tightened down enough, the two broken parts of the cage don't move.

i don't know if i would want to use it for shifting, but i it seems to be perfect as a chain tensioner on a single speed MTB running 48x22 which is low but it's intended to be ridden in the rain so I figured might want a lower gear.